The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to a method and apparatus for creating color transparencies without oil streaks.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
To obviate the foregoing toner offset problem it has been common practice to utilize toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied to the fuser roll surface to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from adhering to the fuser roll surface.
One method of applying a release agent such as silicone oil utilizes a combination donor/metering roll arrangement wherein the metering roll contacts silicone oil in a sump and conveys a metered amount to the donor roll. The metered layer of oil on the metering roll is transported to the donor roll and subsequently to a heated fuser roll. One such Release Agent Metering (RAM) system for applying silicone oils to a heated fuser roll is illustrated in U.S. Pat. No. 4,214,549. The system disclosed therein comprises a donor roll fabricated from a heat insulative and deformable material, for example, silicone rubber which transfers functional release material from a metering roll contacting a supply of release material contained in a sump to the heated fuser roll. A metering blade is supported in contact with the metering roll for metering the release material onto the metering roll to a thickness such that 1 micro liter of oil is dispersed per copy. This type of RAM system dispenses a fixed amount of release agent material to the fuser roll member.
Following is a discussion of other prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
In U.S. Pat. No. 5,200,786 granted to Fromm et al on Apr. 6, 1993, the donor roll of the '549 patent is replaced with a donor brush. As set forth in the '786 patent, the brush donor structure allows for the application of variable amounts of release agent material depending on the mode of operation. In other words, when color prints are being created a greater quantity of silicone oil is applied to the fuser roll compared to the amount applied when operating in the monochrome black mode.
U.S. Pat. No. 4,593,992 issued on Jun. 10, 1986 discloses an image forming apparatus for forming an unfixed image on a recording material including a fixing device having a pair of rotatable members for holding therebetween and conveying the recording material to fix the unfixed image on the recording material, speed control device for variably controlling the fixing rotational speed of the pair of rotatable members to a first fixing speed and a second fixing speed lower than the first fixing speed, application apparatus for intermittently supplying a parting agent to at least one of the pair of rotatable members, and application control apparatus for variably controlling the application acting period of the application apparatus in accordance with the fixing rotational speed of the pair of rotatable members variably set by the speed control device.
U.S. Pat. No. 4,496,234 issued on Jan. 29, 1985 discloses a release agent management (RAM) system for use with a heat and pressure fuser. The system is characterized by the use of a simple reciprocating, positive displacement pump for delivering silicone oil to the heated roll of the fuser. The pump is actuated in response to the fuser rolls being engaged and disengaged, such movement being adapted to act against one or the other of a pair of springs which in cooperation with the oil being pumped forms a damper system which is utilized to control the quantity of oil delivered. The springs and oil cause the velocity of the pump's piston to decay with time which results in more oil being pumped initially.
U.S. Pat. No. 4,047,885 issued on Sep. 13, 1977 discloses contact fuser assembly for use in an electrostatic reproducing apparatus including an internally heated metal core cooperating with a resilient backup roll to form a nip through which substrates carrying toner images are moved with the toner images contacting the metal core. The fuser assembly is characterized by the provision of a sump of liquid release agent material which material is provided for coating the surface of the fuser roll structure. In order to apply the liquid release agent material to the surface of the fuser roll structure there is provided a cylindrical applicator member which is partially submersed in the release agent material. A ratchet wheel and pawl arrangement is provided for periodically indexing or moving the applicator member in response to disengagement of the backup roll from the fuser roll through pivoting of an arm supporting the backup roll. To this end, the pawl member is pivotably supported by the pivot arm and actuates the ratchet wheel each time the pivot arm is moved for effecting disengagement of the backup roll from the fuser roll.
Donor roll RAM systems have the ability to meter oil at a relatively uniform rate over many copies. These oil metering devices are used in all roll fusers that have Viton.TM. as their fusing surface. One problem these RAM systems have is the formation of oil streaks as they age. In the past this, problem was not identified as a copy quality degradation issue when fusing monochrome black images on plain paper. However, color transparencies fused with a fuser utilizing such a RAM system exhibit a severe copy quality problem due to oil streaking. Such streaks of release oil appear as dark lines in the projected color images.